Optical scanner has been widely used in recent years as an input device for image and graphic documents. The diversity of input documents increases constantly. A specially designed optical scanner for transparency is also available now (referring to FIG. 1). Comparing with a conventional optical scanner, there are some major differences which will be described more detailed below.
Light source is disposed in an upper case (10), transparency (11) is placed on a transparent glass (13) of base frame (12) wherein a CCD circuit is held. The movable light source projecting light rays from top downward and transmitting image signals of transparency (11) to CCD circuit in base frame (12). In a conventional optical scanner for hardcopy documents, both light source and CCD circuit are disposed inside of base frame (12).
In a conventional base frame (12) set forth above, the driving mechanism of a movable light source usually adopts a guiding-rod approach (referring to FIG. 2), lamp shade (14) is driven by a driving motor (16) and a belt (17), and makes a linear and reciprocal motion for scanning operation along two guiding frames (15) having been disposed at two lateral sides of base frame (12). The lamp shade moving mechanism set forth above which has been used in conventional optical scanner or copier cannot produce desirable result when applying to a transparency scanner. The reasons and constraints is set forth below.
Firstly, conventional lamp shade moving mechanism consists of many complex components, difficult to maintain or repair, and is costly to produce. Secondly, substantial amount of power and driving torque generated by driving motor would be used to overcome the friction resistance of guiding frame (15), thus makes it difficult to increase the moving speed of the lamp shade moving mechanism, subsequently would not be suitable for a high-speed scanner.